In the practice of many chemical processes proper control of the process requires detection of the presence of a suspended phase in the reaction medium and determination of its concentration, particle size, or both. An example of such a process is one in which crystals are to be formed in a reaction liquid. Detection of the onset of crystallization usually is necessary to control the process in such manner as to yield crystals of the desired size and purity. Detection of the onset of crystallization currently is accomplished by visual monitoring of the reaction medium, detection of the exotherm resulting from the heat of crystallization, or detection of an increase of the turbidity of the reaction medium as a result of the presence therein of crystals. These known methods often lack sufficient sensitivity and dynamic range for proper process control.